Reduce metric writes and preserve scheduler history

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+### 2026.07.13.r011
+- Use durable backup measurements when deciding whether a scheduled job needs a catch-up run, even after activity history was cleared.
+- Append one compact JSON-lines record per completed backup instead of rewriting the full metric history for every queue state change.
+- Migrate the existing `backup-metrics.json` automatically to `backup-metrics.jsonl` and compact in batches after the file exceeds 5,500 records while retaining the newest 5,000.
+
 ### 2026.07.13.r010
 - Return an empty JSON array instead of `null` when no dashboard backup measurements exist yet.
 - Make the WebGUI tolerate empty or invalid metric responses so the rest of the interface remains available.